What is one thing Winston and Julia will not do for the cause ?

During their first visit to O’Brien’s house, Winston and Julia revealed their sentiments towards The Party and its ideals in addition to confessing their allegiance to The Brotherhood and their intention to join it. O’Brien then questioned them on the extent they were willing to go in order to advance the Brotherhood’s course and the two responded that they were willing to do anything including committing murder and suicide among others. However, when asked whether...

During their first visit to O’Brien’s house, Winston and Julia revealed their sentiments towards The Party and its ideals in addition to confessing their allegiance to The Brotherhood and their intention to join it. O’Brien then questioned them on the extent they were willing to go in order to advance the Brotherhood’s course and the two responded that they were willing to do anything including committing murder and suicide among others. However, when asked whether the two of them would be willing to forego their relationship, Julia was quick to intercept Winston’s flow of yes responses and say no. On the other hand, Winston hesitated momentarily but eventually concurred with Julia on the fact that their relationship was the only thing they would not forsake in the name of the Brotherhood. Later on after they are arrested in their secret place, the two were taken to the Ministry of Love where they were tortured and unfortunately betrayed their love for each other. Winston in particular, gave in when threatened by his worst fear, the big hungry rats in Room 101.

By your tag, I see that you are referring to what Julia and Winston say when they go to O'Brien's home to find out more about the Brotherhood. While they are there, O'Brien interrogates them some about what they will be willing to do for the movement. He is trying to give them some idea as to what they will supposedly be required to do.

He asks them whether they are willing to kill (even to kill many innocent people). He asks them if they are willing to die. In all cases, they say they are. But when he asks if they are willing to split up and never see one another again, Julia immediately says no. After a bit, Winston agrees with her and says he is not willing to do this either.